This is a tough one to call.
I used to teach at EGTI when it and EGTH were still open,
and am suprised that there have not been more incidents in this airspace.
With the amount of transit traffic routing around Luton, Stansted and Heathrow CTR's, all capped by the TMA at 2400 feet the only survival strategy I could offer my students was to keep a very good lookout for the majority of people who were obviously not doing so AND get Radar Information Service from the appropriate agency.
The latter was fine in theory, but the frequencies involved were overloaded more often than not because RIS for GA traffic was not their primary role.
There is in my view a case for a dedicated GA RIS in the areas capped by the TMA but this would mean a significant investment by the air traffic service and I cannot see it happening.
These accidents are devastating to the families involved, and there has to be a set of solutions that can, if not eliminate, at least mitigate the risk, if that solution is fag packet sized and AFFORDABLE so be it.